The present invention relates to vertical-cavity surface-emitting lasers and more particular to such lasers having a locally defined sub-wavelength structure for transverse mode and polarization control.
The vertical-cavity surface-emitting laser (VCSEL) is a well established light source in short distance fiber-optic links and interconnects. There is a definite scope for further and more demanding applications, such as in spectroscopy, laser printing, optical storage and longer distance communication. Improvement of laser properties for specific applications would probably yield a larger commercial impact of the VCSEL. For example, in the above-mentioned applications a single mode output power of several milliwatts is often needed, and frequently with a stable linear polarization as an additional requirement.
Due to the relative large transverse extent in combination with a symmetric geometry and isotropic material properties, the VCSEL tends to lase in several transverse modes with an unpredictable state of polarization. The linear polarization states of the individual modes lie in the plane of the epitaxial layers, and due to the electro-optic effect they are normally polarized in the [001] or the [0-11] crystallographic direction. However, the polarization often randomly switches between these two directions because of temperature, injection current, and optical feed-back effects.
Several methods have been developed to obtain single mode emission and/or a stable polarization state from VCSELs, but unfortunately many of these methods negatively affects other important laser characteristics such as the threshold current, differential resistance, and beam quality.
The article entitled “Control of vertical-cavity laser polarization with anisotropic transverse cavity geometries” by K. D. Choquette et al. (Photonics Technology Letters, vol. 6, no. 1, pp. 40-42, January 1994) describes the use of small asymmetric cavity geometries, e.g. rhombus-shaped and dumbbell-shaped, to achieve single mode and polarization stable VCSELs. Special care has to be taken when designing these cavities in order to minimize non-radiative recombination and diffraction losses.
U.S. Pat. No. 6,683,898 provides a method for controlling the mode and polarization state in VCSELs by using photonic band gap structures. If deeply etched photonic crystal structures are used the current injection can be obstructed, resulting in a high differential resistance. Scattering loss, diffraction loss, and non-radiative surface recombination at the etched photonic band gap structures can also affect the laser performance.
The article “Polarization-stable oxide-confined VCSELs with enhanced single-mode output power via monolithically integrated inverted grating reliefs” by J. M. Ostermann et al. (IEEE Journal of Selected Topics in Quantum Electronics, vol. 11, no. 5, pp. 982-989, September/October 2005) describes the use of a locally etched surface grating with a period larger than the optical wavelength in the material. These periods can result in diffraction related losses and beam degradation and the laser performance become very sensitive to the grating geometry and an optimized design therefore requires rigorous electromagnetic modeling.
The present invention uses a sub-wavelength asymmetric polarizing structure, e.g. a grating i.e. a grating with a grating period smaller than the light wavelength in the material, to achieve polarization control and it could also be locally defined to simultaneously achieve transverse mode control. The advantage of using a sub-wavelength grating compared to a larger grating period is that the diffraction related losses and beam degradation are minimized. Moreover, the effective index nature of the sub-wavelength grating will also render the performance rather insensitive to the exact shape and geometry of the grating.
The principle behind the present invention is to control the mode selection and polarization state by introducing a mode and polarization dependent mirror reflectivity/loss in a VCSEL. This is achieved by using a locally defined asymmetric sub-wavelength structure.
In particular the invention provides a vertical-cavity surface-emitting laser (VCSEL) comprising: a bottom mirror structure; a top mirror structure; and an active layer sandwiched between the top mirror structure (100) and the bottom mirror structure. The VCSEL is characterized in that at least one asymmetric sub-wavelength structure is arranged in or at least adjacent to the mirror structure of the VCSEL so as to create a polarization dependent mirror reflectivity from the mirror structure.
Said asymmetric sub-wavelength structure is preferably a grating with a grating period that is smaller than the light wavelength in the grating material.
It is preferred that the grating comprises lines or elongated dots.
It is preferred that said asymmetric sub-wavelength structure is arranged in, or on top of, or on the bottom of the top mirror structure of the VCSEL.
However, said sub-wavelength structure can alternatively be arranged in, or on top of, or at the bottom of the bottom mirror structure of the VCSEL.
Said asymmetric sub-wavelength structure can be locally defined in or adjacent to an area of the VCSEL wherein a transverse mode has a large intensity, so as to achieve transverse mode control.
An epitaxial layer in the epitaxial structure of said VCSEL can be partly oxidized to yield an oxide aperture 300; in which case it is preferred that the diameter or cross-section of the grating region is smaller than the diameter or cross-section of the oxide aperture 300.
Said asymmetric sub-wavelength structure can be defined in a substantially λ/4-thick layer arranged on the top mirror structure, or in a substantially λ/4-thick layer arranged on the bottom mirror structure of said VCSEL.
Said asymmetric sub-wavelength structure can be defined in a top layer of the top mirror structure, or in a bottom layer of the bottom mirror structure.
The duty cycle of the of said grating period is preferably arranged so that a large anisotropy in effective index is achieved between a direction perpendicular to the grating lines or grating dots and a direction parallel to the grating lines or grating dots.
Said sub-wavelength structure is preferably made of one of semiconductor material, dielectric material or metal material.
In particular, the invention provides for of a vertical-cavity surface-emitting laser (VCSEL) that can be used in spectroscopy applications, optical communication, optical data storage, laser printers, optical mouse, free-space interconnects, measurements techniques, which VCSEL comprises: a bottom mirror structure; a top mirror structure; and an active layer sandwiched between the top mirror structure and the bottom mirror structure; and
at least one asymmetric sub-wavelength structure arranged in or at least adjacent to the mirror structure of said VCSEL so as to create a polarization dependent mirror reflectivity from said mirror structure.
In particular, the invention provides for of a vertical-cavity surface-emitting laser (VCSEL) that can be used in spectroscopy applications, optical communication, optical data storage, laser printers, optical mouse, free-space interconnects, measurements techniques, which VCSEL comprises: a bottom mirror structure; a top mirror structure; an active layer sandwiched between the top mirror structure and the bottom mirror structure; and
at least one asymmetric sub-wavelength structure (106; 200; 301; 304) locally arranged in or at least adjacent to the mirror structure of said VCSEL so as to create a polarization and transverse mode dependent mirror reflectivity from said mirror structure.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
In the following the invention will be described in a non-limiting way and in more detail with reference to exemplary embodiments illustrated in the enclosed drawings, in which:
FIGS. 2(a)-2(c) illustrate schematic top views of three asymmetric sub-wavelength structures that can be used for polarization control according to embodiments of the present invention;
FIGS. 3(a)-3(b) illustrate schematic top and cross-sectional views of two VCSEL designs according to embodiments of the present invention;
FIGS. 5(a)-5(b) illustrate schematic top and cross-sectional views of two VCSEL designs according to embodiments of the present invention;
FIGS. 6(a)-6(b) show the calculated polarization dependent modal loss as a function of grating duty cycle and depth for two designs according to an embodiment of the invention;
FIGS. 7(a)-7(b) show measured output power (polarization resolved) and OPSR versus current, optical spectra, and far-field, for two VCSELs according to an embodiment of the invention;
The sub-wavelength structure act as an effective index medium, and by an asymmetric design, a high contrast in effective index can be achieved between two orthogonal polarization states. The sub-wavelength structure is included in the mirror structure of the VCSEL and as a result the two orthogonal polarization states will experience two different mirror reflectivities. For one polarization state, the reflectivity contribution from the sub-wavelength structure will be more in phase with the rest of the reflections in the mirror stack, producing a high mirror reflectivity, while for the orthogonal polarization state, the contribution will be more out-of-phase, reducing the total mirror reflectivity. As a result, the polarization state with a lower mirror reflectivity will be suppressed. Thus, a polarization stable laser can be achieved by applying an asymmetric sub-wavelength structure over the emission window of the laser.
Many different asymmetric sub-wavelength structures can be used to achieve polarization control, where two examples are given in
The sub-wavelength structure can be made of any suitable material for example semiconductor material, dielectrics and metals. It can be defined in the top layer/layers of a mirror structure and/or in layers deposited on top of mirror structure and/or defined in one or several of the mirror layers further down in the mirror stack in the top and/or bottom mirror.
The asymmetric sub-wavelength structure can be applied to emission wavelengths between 100 nm and 10 μm. If the asymmetric structure is in the form of a grating as suggested above, the period should be smaller than the light wavelength in the grating material. For example, if the emission wavelength is 850 nm and the effective index of refraction of the grating structure is 3.5, the light wavelength in the material becomes 850/3.5=243 nm, i.e. the period of the grating should be smaller than 243 nm.
In
In
In
If the asymmetric sub-wavelength structure is applied locally, not only polarization control but also mode control can be achieved. In the region where a sub-wavelength structure is defined the mirror reflectivity is affected. By defining the structure locally, in areas where a certain transverse mode/modes has a large intensity, this mode/modes is mainly affected and can be suppressed or enhanced, thus a mode selectivity can be achieved.
In
The calculated polarization dependent modal loss for the conventional structure 401 and the inverted structure 402 are shown in FIGS. 6(a) and (b) respectively. For a depth 310 of 60 nm and a duty cycle of 60% a >20 cm−1 mode selectivity, i.e. loss difference between the fundamental-mode (LP01) and the first higher-order mode (LP11), and simultaneously a 15 cm−1 polarization selectivity, i.e. loss difference between LP01-E∥ and LP01-E⊥, can be achieved in both cases. In the conventional 401 technique LP01-E∥ is favoured, while in the inverted 402 technique it is LP01-E⊥. Further, in the conventional 401 technique the polarization selectivity is larger than the mode selectivity while in the inverted 402 technique it is the opposite. The dependence on depth 310 and duty cycle is also different between the conventional 401 and the inverted 402 technique. In the inverted 401 case a depth variation of ±20 nm from the targeted depth of 60 nm will still maintain the high values of mode and polarization selectivity without significantly increasing the loss for the favoured LP01-E⊥, while in the conventional 402 case the dependence on depth 310 is much stronger and the loss for the favoured fundamental mode also changes dramatically with depth 310. Thus, a small performance variation in threshold current, output power, and mode and polarization selectivity over a broad range of depths 310 can only be anticipated for the inverted 402 technique. Comparing the modal loss as a function of duty cycle for the conventional 401 and inverted 402 technique it can be seen that for the conventional 401 case a high mode selectivity of >15 cm−1 and a polarization selectivity of 10 cm−1 can be obtained within a larger range of duty cycles. However, the modal loss for the favoured mode has a much stronger duty cycle dependence than in the inverted 402 case, which leads to a larger variation in threshold current, slope efficiency, resonance frequency etc. If only a small performance variation is allowed the duty cycle range for the inverted technique 402 is much larger than for the normal 401 technique. In other words, the inverted sub-wavelength surface grating technique is preferable since a large mode and polarization selectivity can be achieved over a broad range of depths and duty cycles without negatively affecting the performance, which in turn facilitates the fabrication.
Applications of these sub-wavelength-structured VCSELs include spectroscopy applications where a single mode and polarization stable VCSEL are of utmost importance to be able to measure a single or several spectroscopic lines. In addition, the sub-wavelength structured VCSELs can be used in optical communication, e.g. as transmitters for local and storage area networks where single mode and polarization stable operation is desired, as well as in optical data storage and optical pumping. Furthermore, the sub-wavelength-structured VCSELs can be used as transmitters in applications where a good beam quality is needed such as in a laser printer, an optical mouse, and a free-space optical interconnect. They can also be used as a transmitter in a number of different measurements techniques which profit from single mode emission and good beam quality such as in Doppler-based and interference-based measurement techniques.
The sub-wavelength grating structure can be formed in a number of different ways. The structure can be defined by nano-imprint, electron beam lithography, or other lithography techniques capable of defining structures in the nanometer range. The structure can then be transferred into the intended material by wet etching or dry etching techniques. Another possibility is to use standard methods for material deposition and lift-off to form the sub-wavelength structure.
It should be noted that the word “comprising” does not exclude the presence of other elements or steps than those listed and the words “a” or “an” preceding an element do not exclude the presence of a plurality of such elements. It should further be noted that any reference signs do not limit the scope of the claims, and that several “means”, “devices”, and “units” may be represented by the same item of hardware.
The above mentioned and described embodiments are only given as examples and should not be limiting to the present invention. Other solutions, uses, objectives, and functions within the scope of the invention as claimed in the below described patent claims should be apparent for the person skilled in the art.